This invention relates to hydrophone arrays and more particularly to a multi-layered piezoelectric polymer (PVDF) hydrophone array.
In the prior art, piezoelectric ceramics, particularly the lead zirconate titanates (PZT), have been the most commonly used transduction materials in both hydrophones and projectors. Piezoceramic has offered considerable flexibility in hydrophone design, because it can be made in a wide variety of shapes, such as cylinders, rings, plates and hemispheres. While ceramic can be made in different forms, there are practical constraints on the maximum size that can be fabricated in one piece. This is a consequence of the hard and brittle nature of piezoceramic materials. The constrained maximum size, ceramic rigidity, and its high mass density--which is almost equal to the denisty of steel--are disadvantages in some applications; for example, towed arrays and large aperture hull-mounted arrays used on ships.
Piezoelectric polymer (PVDF), a recent development in new transduction materials, overcomes these limitations and has led to the development of light weight, flexible hydrophones. As employed in this invention, PVDF is capable of being fabricated as long strips for application in towed arrays or large flat sheets for use in large aperture arrays. The lower density and mechanical flexibility of polymer have already proved to be advantageous in its use in thin line towed arrays, where the hydrophones must be long, thin, flexible and light weight. A long length hydrophone reduces the noise associated with turbulent boundary layer phenomenon by the method of spatial averaging. This method takes advantage of the slower speed of flow noise than the signal's sound speed which results in turbulent boundary layer noise--but not signal--descrimination. The hydrophone elements of large aperture arrays must also be of large lateral dimensions, light weight, thin, semi-flexible, and capable of descriminating against high wave number turbulent boundary layer noise. Hull-mounted large aperture array hydrophones must also be capable of descriminating against medium-wave number hull vibration noise as in the present invention.
Because of its light weight and mechanical flexibility, polymer is more shock resistant than piezoceramic. Also, the mechanical flexibility of polymer allows the material to conform to irregular surfaces and makes possible new types of hydrophone designs. In addition, the characteristic impedance of polymer more nearly matches that of water. The polymer also has the advantage of having large piezoelectric stress constants.
Piezoelectric polymer film is presently made of polyvinylidene fluoride and is often referred to as PVDF. A polarization procedure must be used to render the polymer usefully piezoelectric. This procedure comprises uni-axially stretching the film at elevated temperatures to several times its original length. In one method of polarization, both surfaces of the film are metallized and a high DC electric field is applied to the electrodes and held for about one hour at 100.degree. C. Subsequent cooling to room temperature under the applied field results in permanent polarization.